CVD processes for synthesis of diamond material are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R. S Balmer et al. gives a comprehensive overview of CVD diamond materials, technology, and applications (see “Chemical vapour deposition synthetic diamond: materials, technology and applications” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364221).
Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen for CVD synthetic diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma. Various aspects of a microwave plasma activated CVD reactor adapted for diamond synthesis are described in the patent literature including WO 2012/084657, WO 2012/084655, WO 2012/084658, WO 2012/084659, WO 2012/084660, and WO 2012/084661.
In the presence of a suitable substrate material, CVD synthetic diamond material can be deposited. Polycrystalline CVD diamond material may be formed on a non-diamond substrate, typically formed of a carbide forming material such as silicon, silicon carbide, or refractory metals such as molybdenum, tungsten, titanium, etc. Single crystal CVD synthetic diamond material may be formed by homoepitaxial growth on a single crystal diamond substrate. There are several advantages to single crystal CVD diamond material for certain applications due to the avoidance of grain boundaries, e.g. higher thermal conductivity for thermal heat spreading applications and lower scattering of light for certain optical applications. However, to date single crystal CVD diamond material is only available in relatively small sizes and thus for many applications polycrystalline CVD diamond components are still preferred, e.g. for large area optical windows and heat spreaders. It has also been proposed to combine the more extreme characteristics of single crystal CVD diamond material with large area polycrystalline CVD diamond wafers by providing composite wafers comprising a plurality of single crystal diamond substrates bonded to a polycrystalline CVD diamond carrier wafer. Such composite substrates are described in WO 2005/010245 and comprise a polycrystalline CVD diamond support layer and a plurality of single crystal diamond substrates fixed to the polycrystalline CVD diamond support layer. Device structures can then be fabricated on the plurality of single crystal diamond substrates. Various ways of bonding the single crystal diamond substrates to the polycrystalline CVD diamond support layer are described in WO 2005/010245 including the use of adhesives such as gluing or brazing. WO 2005/010245 also indicates that a preferred bonding method is direct diamond-to-diamond bonding by growing the polycrystalline CVD diamond support layer directly onto an array of single crystal diamond substrates. For example, WO 2005/010245 suggests that single crystal diamond substrates can be brazed to a backing wafer such as silicon, tungsten or polycrystalline diamond and a layer of polycrystalline CVD diamond grown thereon. Subsequently the backing wafer can be retained or removed, for example, to provide a polycrystalline CVD diamond wafer in which a plurality of single crystal diamond substrates are disposed with both surfaces of the single crystal diamond substrates exposed, e.g. to provide optical windows.
Having regard to single crystal CVD diamond growth, it is commercially advantageous to synthesize a plurality of single crystal CVD diamonds in a single growth run. A plurality of single crystal CVD synthetic diamonds can be fabricated in a single CVD growth run by providing a plurality of single crystal diamond substrates on a carrier substrate. The carrier substrate is typically formed of a carbide forming material such as silicon, silicon carbide, or refractory metals such as molybdenum, tungsten, titanium, etc. For example, the substrates can be placed on a refractory metal carrier substrate or bonded thereto by soldering or brazing. One problem with this approach to synthesizing a plurality of single crystal CVD diamonds is that of uniformity and yield. Non-uniformities can exist in terms of crystal morphology, growth rate, cracking, and impurity content and distribution. For example, as described in WO2013/087697, even if the CVD diamond growth chemistry is carefully controlled, non-uniform uptake of impurities can still occur due to temperature variations at the growth surface which affect the rate of impurity uptake. Variations in temperature also cause variations in crystal morphology, growth rate, and cracking issues. These temperature variations can be in a lateral direction relative to the growth direction at a particular point in the growth run (spatially distributed) or parallel to the growth direction due to variations in temperature over the duration of a growth run (temporally distributed). Variations can occur within a single CVD diamond stone and also from stone to stone in a multi-stone synthesis process. As such, in a multi-stone synthesis process only a portion of product diamond stones from a single growth run may meet a target specification. In this regard, WO2013/087697 discusses brazing of single crystal substrates to a refractory metal carrier substrate and some suitable braze alloys to obtain a good adhesion and thermal contact between the single crystal diamond substrates and the underlying refractory metal carrier substrate to improve uniformity and yield of single crystal CVD synthetic diamond product.
In addition to the above, contamination of the single crystal CVD diamond product stones can result as material from the carrier substrate is etched away and becomes incorporated into the single crystal CVD diamond material during growth. In this regard, it may be noted that impurities in the CVD processes are critical to the type of diamond material which is produced. For example, various impurities may be intentionally introduced into the CVD process gases, or intentionally excluded from the CVD process gases, in order to engineer a CVD synthetic diamond material for a particular application. Furthermore, the nature of the substrate material and the growth conditions can affect the type and distribution of defects incorporated into the CVD synthetic diamond material during growth. Patent literature describing various types of single crystal CVD diamond material and methods of fabrication include WO01/096633, WO01/096634, WO2004/046427, WO2007/066215, WO 2010010344, WO 2010010352, WO03/052174, WO2003/052177, WO2011/076643, and WO2013/087697.
In light of the above, it is clear that effective thermal management and control of impurities are thus key features to achieving uniform single crystal CVD diamond material at high yields according to a target specification. It is an aim of embodiments of the present invention to address these issues and provide an improved single crystal CVD diamond growth process.